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2 x 1ph cables used as 3ph...

tonydon
Ok probably stupid question

I'm pretty sure this cant be done .. 


We have 2 x 35mm 1ph armoureds feeding a building..

the cables run through ducting next to each other.


The building is to be modified and requires 1 x 3ph supply...


Is it against regs to use one cable as  L1 , N , E

And the second cable as  L2, L3, E         Or     L2, L3, N




  • 0
    It would fail on 521.5 because of the armour.


  • 0
    It can Tony, but there could be a couple of problems.

    What is the load you have? Is it several 3ph loads, or a number of single phase ones? Are the phase loads balanced in any way or mixed single and 3 phase?

    The reason for these questions is eddy currents and unbalanced magnetic fields in the armour which could cause significant heating.

    If the cables are in a duct I suggest the easiest thing is to pull out the old cables and replace them with a new 4 core one, but this could be expensive. Let's have the answers and then consider the problem again.


  • 0
    L1 L2  and N for L1 and L2

    in one 3 core and

    L3, N for L3 only and an extra  CPC  in the other might do if you can avoid the wrong neutral curreets sharing a core, and the armour is low Z enough to be the main CPC.

    However, not if the loads really are 3 phase, or the neutrals from the 3 phases are  mixed. (so not just 3 single phase boards with a separated neutrals) then no you cannot, as the 'flow' and 'return' currents in side any one  SWA must sum to more or less zero or it will run hot.

    (and for this reason the regs do not like it)

    Mike.
  • 0
    There's also a recent general requirement not to distribute the conductors of a circuit over different multicore cables (with some get-outs for conductors in parallel) - 521.8.1 - and that one applies even when there's no ferrous enclosure (so you can't use two T&Es for 3-phase either).

       - Andy.
  • 0
    Many Thanks everyone...


    Pretty much what we thought


    3 sparks 110 years experience and still wasnt sure.....


    The regs are far too complex for us old guys... lol

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